Isobutene is an important organic chemical material, and high purity isobutene is widely used for producing products such as butyl rubbers, polyisobutenes, and so on. Among numerous preparation methods, the method for preparing high purity isobutene by dissociation of methyl tert-butyl ether (MTBE) is advanced in technology and economy. The by-product dimethyl ether in dissociation of MTBE is the main factor that affects purity of the product isobutene. When the product isobutene is used for producing butyl rubbers, there are strict requirements on the content of dimethyl ether. Therefore, the core of this technology is to develop catalysts that can secure high dissociation conversion of MTBE and excellent selectivity of isobutene, with as little as possible or even no generation of dimethyl ether, so as to reduce burden on subsequent separation in MTBE crackers.
Currently, catalysts used for dissociation of MTBE with better performances are alumina catalysts, silica catalysts, ion exchange resin catalysts (as disclosed in DE 3509292A1, DE 3210435A1, U.S. Pat. No. 4,447,668A, GB 1482883A, U.S. Pat. No. 4,570,026A, and U.S. Pat. No. 4,551,567A), sulfate catalysts (as disclosed in JP7626401), active carbon catalysts (as disclosed in JP 7494602), solid phosphoric acid catalysts (as disclosed in CN 96123535.7 and EP 0118085A1), etc.
Comprehensive consideration from the aspects of activity, stability, possibility of regeneration, cost of the catalysts, and so on, arrives at a fact that adopting alumina or silica catalysts for dissociation of MTBE is a preferred selection. When alumina catalysts are used, owing to existence of a large amount of alumina, the catalyst activity is decreased, and therefore a high reaction temperature is required. Especially, methanol generated in the dissociation would be dehydrated under the reaction conditions to produce dimethyl ether, which does not only reduce a yield of methanol, but also negatively affect separation of the products.
In order to reduce or eliminate generation of dimethyl ether, selecting suitable catalysts such as silica based catalysts, or catalysts containing both alumina and silica, is the main approach for reducing by-products of MTBE dissociation, in addition to adopting water injection technology in the reaction process.
The catalysts disclosed in CN 96115213.3 are prepared by calcining silica gel at a temperature in a range from 350 to 550° C. for 1 to 6 h, and then hydrothermally treating the silica gel at a temperature in a range from 200 to 400° C. with saturated steam for 1 to 6 h. Under the reaction conditions of a temperature in a range from 180 to 260° C., a pressure in a range from 0.1 to 0.8 MPa, and an MTBE WHSV (weight hourly space velocity) in a range from 2 to 6 h−1, the catalysts can lead to high conversion of MTBE and high selectivity of isobutene, but at the same time with an amount of the by-product dimethyl ether being 0.30%.
In an article titled “Research and Development of Catalysts for Preparing Isobutene by Dissociation of MTBE” (Liu Fusheng, et al.), under “Silica catalytic system” (see Speciality Petrochemicals, 1997, No. 4, Pages 1 to 4), it discloses that when a silica based catalyst is adopted, the catalyst hardly has any catalytic activity in case of silica being separately used. However, although addition of a small amount of alumina or modification of the catalyst with a small amount of alumina can increase activity and selectivity of the silica catalyst, the purities of silica raw materials will significantly influence performances of the catalyst. For example, WO 8700166A1 discloses obtaining modified catalysts by impregnating soluble aluminum salt solutions on a high purity silicon oxide, and then drying and calcining the resulting mixture.
An article titled “Preparation of High Purity Isobutene by Dissociation of MTBE” (see Journal of Fuel Chemistry and Technology, 2003, Vol. 31, No. 2, Pages 156 to 160) presents a silica-alumina catalyst with a silica amount ranging from 79 wt % to 89 wt %, an alumina amount ranging from 9 wt % to 12 wt %, and an amount of other additives ranging from 10 wt % to 12 wt %. This silica-alumina catalyst is prepared by the following method: firstly, a silica-alumina sol is prepared and neutralized with ammonia water, which is then added with promoters after being aged and eliminated impurities by washing; next, a resulting mixture obtained in the first step is treated (the treating steps are not disclosed herein), molded, dried, and calcined at 850° C. to obtain the catalyst. In the above-mentioned method, the calcining temperature is higher than 700° C., so that the activity and selectivity of the catalyst can reach an optimal state. When the calcining temperature is lower than 700° C., reaction activity of side reactions such as isobutene polymerization or methanol dehydrating of the obtained catalyst, and the like would be increased and thus affect selectivity of the catalyst. In addition, depositions of polymers in the by-products can also affect service life of the catalyst.
CN 1853772A discloses a modified silica-alumina catalyst used for preparation of isobutene by dissociation of MTBE and preparation thereof. This catalyst is obtained by treating amorphous silica-alumina with saturated steam. Because of a smaller specific surface area of the amorphous silica-alumina used separately and a relatively high amount of the L acid of the weak acid, not only improvement of reactivity and selectivity of the catalyst can be affected, but also the content of the by-product dimethyl ether in dissociation of MTBE is still very high.